Typical materials requiring surface flatness at high level include a single-crystal silicon disk called a silicon wafer for producing semiconductor integrated circuits (IC, LSI). The surface of the silicon wafer should be flattened highly accurately in a process of producing IC, LSI etc., in order to provide reliable semiconductor connections for various coatings used in manufacturing the circuits. In the step of polishing finish, a polishing pad is generally stuck on a rotatable supporting disk called a platen, while a workpiece such as a semiconductor wafer is stuck on a polishing head. By movement of the two, a relative speed is generated between the platen and the polishing head while polishing slurry having abrasive grains is continuously supplied to the polishing pad, to effect polishing processing.
As polishing characteristics of a polishing pad, it is requested that a polished object is excellent in planarity and in-plane uniformity and a polishing rate is large. A planarity and in-plane uniformity of a polished object can be improved to some extent with a polishing layer higher in elastic modulus. A polishing rate can be bettered by increasing a holding quantity of a slurry on a foam with cells therein.
Polishing pads including a polyurethane foam are proposed as polishing pads that meet the above properties (see Patent Documents 1 and 2). Such a polyurethane foam is produced by a reaction of an isocyanate-terminated prepolymer with a chain extender (curing agent), in which in view of hydrolysis resistance, elastic properties, wear resistance, or the like, a polyether (a polytetramethylene glycol with a number average molecular weight of 500 to 1,600) or a polycarbonate is preferably used as a high molecular polyol component for the isocyanate prepolymer.
However, when the above polishing layer absorbs moisture or water, the cohesion of its hard segment can be reduced so that its dimensional stability can be easily reduced. The polishing pad also has a problem in which in serious cases, it is warped or heaved so that its polishing properties such as planarization properties and in-plane uniformity may gradually change.
Patent Document 3 discloses that in order to improve the retainability slurry, a polymer composition for polishing pads should show a volume swelling rate of 20% or less when it is immersed in water at 23° C. for 72 hours. However, such a polymer composition for polishing pads uses a thermoplastic polymer and thus can hardly form a polishing pad that can maintain a high level of dimensional stability when it absorbs moisture or water.
Patent Document 1: Japanese Patent Application Laid-Open (JP-A) No. 2000-17252
Patent Document 2: Japanese Patent No. 3359629
Patent Document 3: JP-A No. 2001-47355